Circulatory system a. Human circulatory system. Circle of the portal vein

Among the main systems included in the human body, the circulatory system occupies a special place. How the circulatory system works remained a mystery to scientists until the 16th century. Such outstanding thinkers as Aristotle, Galen, Harvey and many others worked on its solution. All their discoveries are summarized into a coherent system of anatomical and physiological concepts.

Historical reference

A special role in the formation of correct ideas about what organs the human circulatory system consists of was played by the Spanish scientist Servetus and the English naturalist William Harvey. The first was able to prove that blood from the right ventricle can enter the left atrium only through the lung network. Harvey discovered the so-called great circle (closed) blood circulation. This put an end to the question of whether blood moves strictly in a closed system or not. The circulatory system of humans and mammals is closed.

It is also necessary to recall the works of the Italian physician Malpighi, who discovered capillary circulation. Thanks to his research, it became clear how it turns into venous and vice versa. How does anatomy address this issue? The human circulatory system is a collection of organs such as the heart, blood vessels and auxiliary organs - red bone marrow, spleen and liver.

The heart is the main organ of the human circulatory system

Since ancient times, in all cultures without exception, the heart has been assigned a central role not only as an organ of the physical body, but also as the spiritual container of a person’s personality. In the expressions “friend of the heart”, “from the bottom of my heart”, “sorrow in my heart”, people showed the role of this organ in the formation of emotions and feelings.

Liquid tissue in the human body

The functions of transporting oxygen and nutrients, removing waste and toxins, and producing antibodies are performed by the circulatory system. Blood, the structure of which can be represented as a mixture of cells (leukocytes, erythrocytes and platelets) and plasma (liquid part), ensures the fulfillment of the above tasks.

In the human body there are hematopoietic tissues, one of which is myeloid. It is the leading one in the red bone marrow, located in the diaphysis and contains the precursors of erythrocytes, leukocytes and platelets.

Features of the structure of blood

The red color of blood is due to the presence of hemoglobin pigment. It is responsible for the transport of gases dissolved in the blood - oxygen and carbon monoxide. It can have two forms: oxyhemoglobin and carboxyhemoglobin. 90% consists of water.

The remaining substances are proteins (albumin, fibrinogen, gamma globulin) and mineral salts, the main of which is sodium chloride. The formed elements of blood perform the following functions:

• red blood cells - carry oxygen;
• leukocytes, or white blood cells (neutrophils, eosinophils, T-lymphocytes, etc.) - participate in the formation of immunity;
• platelets - help stop bleeding when the integrity of the vessel walls is violated (responsible for blood clotting).

The human circulatory system, due to the various functions of blood, is the most important in maintaining homeostasis of the body.

Vessels of the body: arteries, veins, capillaries

To understand what organs the human circulatory system consists of, you need to imagine it as a network of tubes with different diameters and wall thicknesses. Arteries have a powerful muscular wall, since blood moves through them at high speed and high pressure. Therefore, arterial bleeding is very dangerous, as a result of which a person loses a large amount of blood in a short time. This could have fatal consequences.

The veins have soft walls, abundantly supplied with semilunar valves. They ensure the movement of blood in the vessels in only one direction - to the main muscular organ of the circulatory system. Since venous blood must overcome gravity to rise to the heart, and the pressure in the veins is low, these valves prevent blood from flowing backwards, away from the heart.

A network of capillaries with microscopic wall diameters performs the main function of gas exchange. It is in them that carbon dioxide (carbon dioxide) and toxins from tissue cells enter, and capillary blood, in turn, gives the cells the oxygen necessary for their life. In total, the body contains more than 150 billion capillaries, the total length of which in an adult is approximately 100 thousand km.

A special functional adaptation of the human body that ensures a constant supply of organs and tissues with necessary substances, which can be observed both in physiologically normal conditions and in case of complex disruptions of the system (for example, blockage of a vessel with a blood clot).

Systemic circulation

Let us return to the question of what organs the human circulatory system consists of. Let us recall that the vicious circle of blood circulation, discovered by Harvey, originates in the left ventricle and ends in the right atrium.

The aorta, as the main artery in the body and the beginning of the systemic circulation, carries oxygenated blood from the left ventricle. Through a system of vessels extending from the aorta and branching throughout the human body, blood flows to all parts of the body and organs, saturating them with oxygen, performing the functions of exchange and transport of nutrients.

From the upper part of the body (head, shoulders, chest, upper limbs), venous blood, saturated with carbon dioxide, is collected in and from the lower half of the body - in the inferior vena cava. Both vena cava flow into the right atrium, closing the systemic circulation.

Pulmonary circulation

The circulatory system - the heart, the circulatory system - are also included in the so-called pulmonary (pulmonary) circulation. It was discovered by Miguel Servet in the mid-16th century. This circle starts from the right ventricle and ends at the left atrium.

Venous blood flows through the right atrioventricular orifice from the right atrium into the right ventricle. From it along the pulmonary trunk, and then through two pulmonary arteries - left and right - it enters the lungs. And despite the fact that these vessels are called arteries, the blood flows through them is venous. It enters the right and left lungs, in which there are capillaries that encircle the alveoli (pulmonary vesicles that make up the lung parenchyma). Gas exchange occurs between the oxygen of the alveoli and the connective tissue through the thinnest walls of the capillaries. It is in this part of the body that venous blood turns into arterial blood. Then it enters the postcapillary venules, which enlarge to 4 pulmonary veins. Through them, arterial blood enters the left atrium, where the pulmonary circulation ends.

Blood circulation through all vessels occurs simultaneously, without stopping or interrupting for a second.

Coronary circulation

What the autonomous circulatory system is, what organs it consists of, and what are the features of its functioning have been studied by scientists such as Shumlyansky, Bowman, and Gis. They found that the greatest importance in this system is the coronary or coronary blood circulation, which is carried out by special blood vessels that encircle the heart and extend from the aorta. These are vessels such as the left coronary artery with the main branches, namely: the anterior interventricular, circumflex branch and atrial branches. It is also the right coronary artery with the following branches: right coronary and posterior interventricular.

Blood without oxygen returns back to the muscular organ in three ways: through the coronary sinus, veins entering the atrium, and the smallest vascular branches that flow into the right half of the heart without even appearing on its epicardium.

Circle of the portal vein

Since the circulatory system is very important in ensuring the internal constancy of the environment, what organs the portal vein circle consists of, natural scientists studied in the process of considering the systemic circulation. It was found that blood from the gastrointestinal tract, spleen and pancreas accumulates in the inferior and superior mesenteric veins, which subsequently unite to form the portal vein.

The portal vein, together with the hepatic artery, enters the portal of the liver. Arterial and venous blood in hepatocytes (liver cells) undergoes thorough cleaning and then enters the right atrium. Thus, blood purification occurs due to the barrier function of the liver, which is also provided by the circulatory system.

What organs does the auxiliary system consist of?

The auxiliary organs include red bone marrow, spleen and the aforementioned liver. Since blood cells do not live long, approximately 60-90 days, there is a need to utilize old waste blood cells and synthesize new ones. It is these processes that provide the auxiliary organs of the circulatory system.

In the red bone marrow, which contains myeloid tissue, precursors of formed elements are synthesized.

The spleen, in addition to its function of depositing part of the blood not used in circulation, destroys old red blood cells and partially replenishes their loss.

The liver also disposes of dead leukocytes, red blood cells and platelets and stores blood that is not currently involved in the circulatory system.

The article examined in detail the circulatory system, what organs it consists of and what functions it performs in the human body.

Blood- liquid tissue that circulates in the human circulatory system and is an opaque red liquid consisting of pale yellow plasma and cells suspended in it - red blood cells (erythrocytes), white blood cells (leukocytes) and red platelets (platelets). The share of suspended cells (shaped elements) accounts for 42–46% of the total blood volume.

The main function of blood is the transport of various substances within the body. It transports respiratory gases (oxygen and carbon dioxide) both physically dissolved and chemically bound. Blood has this ability thanks to hemoglobin, a protein contained in red blood cells. In addition, blood carries nutrients from the organs where they are absorbed or stored to the point of consumption; the metabolites (metabolic products) formed here are transported to the excretory organs or to those structures where their further use can occur. Hormones, vitamins and enzymes are also transported purposefully to target organs by the blood. Due to the high heat capacity of its main component - water (1 liter of plasma contains 900-910 g of water), blood ensures the distribution of heat generated during the metabolic process and its release into the external environment through the lungs, respiratory tract and skin surface.

The proportion of blood in an adult is approximately 6–8% of the total body weight, which corresponds to 4–6 liters. A person’s blood volume can undergo significant and long-term deviations depending on the degree of training, climatic and hormonal factors. Thus, in some athletes, blood volume as a result of training can exceed 7 liters. And after a long period of bed rest, it may become lower than normal. Short-term changes in blood volume are observed during the transition from a horizontal to a vertical position of the body and during muscle load.

Blood can perform its functions only by being in constant motion. This movement is carried out through a system of blood vessels (elastic tubes) and is provided by the heart. Thanks to the body's vascular system, blood is available to all corners of the human body, every cell. The heart and blood vessels (arteries, capillaries, veins) form cardiovascular system (Fig. 2.1).

The movement of blood through the vessels of the lungs from the right heart to the left is called pulmonary circulation (pulmonary circle). It begins with the right ventricle, which ejects blood into the pulmonary trunk. Then the blood enters the vascular system of the lungs, which has in general the same structure as the systemic circulation. Further, through four large pulmonary veins it flows to the left atrium (Fig. 2.2).

It should be noted that arteries and veins differ not in the composition of the blood moving in them, but in the direction of movement. So, blood flows through the veins to the heart, and through the arteries it flows away from it. In the systemic circulation, oxygenated (oxygen-enriched) blood flows through the arteries, and in the pulmonary circulation through the veins. Therefore, when oxygenated blood is called arterial, only systemic circulation is meant.

Heart is a hollow muscular organ divided into two parts - the so-called “left” and “right” heart, each of which includes an atrium and a ventricle. Partially deoxygenated blood from the organs and tissues of the body flows to the right heart, which pushes it out to the lungs. In the lungs, the blood is saturated with oxygen, partially deprived of carbon dioxide, then returns to the left heart and again enters the organs.

The pumping function of the heart is based on the alternation of contraction (systole) and relaxation (diastole) of the ventricles, which is possible due to the physiological characteristics of the myocardium (muscular tissue of the heart, constituting the bulk of its mass) - automaticity, excitability, conductivity, contractility and refractoriness. During diastole the ventricles fill with blood, and during systole they throw it into large arteries (aorta and pulmonary trunk). At the exit of the ventricles there are valves that prevent blood from flowing back from the arteries into the heart. Before filling the ventricles, blood flows through the large veins (caval and pulmonary) into the atria.

Rice. 2.1. Human cardiovascular system

Atrial systole precedes ventricular systole; thus, the atria serve as auxiliary pumps that help fill the ventricles.

Rice. 2.2. The structure of the heart, small (pulmonary) and systemic circulation

The blood supply to all organs (except the lungs) and the outflow of blood from them is called the systemic circulation (great circle). It begins with the left ventricle, which ejects blood into the aorta during systole. Numerous arteries depart from the aorta, through which the blood flow is distributed into several parallel regional vascular networks that supply blood to individual organs and tissues - the heart, brain, liver, kidneys, muscles, skin, etc. The arteries are divided, and as their number increases the diameter of each of them decreases. As a result of the branching of the smallest arteries (arterioles), a capillary network is formed - a dense interweaving of small vessels with very thin walls. This is where the main two-way exchange of various substances between blood and cells occurs. When capillaries merge, venules are formed, which then combine to form veins. Ultimately, only two veins approach the right atrium - the superior vena cava and the inferior vena cava.

Of course, in fact, both circles of blood circulation form a single bloodstream, in two sections of which (the right and left heart) the blood is imparted kinetic energy. Although there is a fundamental functional difference between them. The volume of blood released into the systemic circle must be distributed among all organs and tissues, the need for blood supply is different and depends on their condition and activity. Any changes are instantly registered by the central nervous system (CNS), and the blood supply to the organs is regulated by a number of control mechanisms. As for the vessels of the lungs, through which a constant amount of blood passes, they place relatively constant demands on the right heart and mainly perform the functions of gas exchange and heat transfer. Therefore, the pulmonary blood flow regulation system is less complex.

In an adult, approximately 84% of all blood is contained in the systemic circulation, 9% in the pulmonary circulation, and the remaining 7% directly in the heart. The largest volume of blood is contained in the veins (approximately 64% of the total blood volume in the body), i.e. veins play the role of blood reservoirs. At rest, blood circulates in only about 25–35% of all capillaries. The main hematopoietic organ is the bone marrow.

The demands placed on the circulatory system by the body vary significantly, so its activity varies widely. Thus, at rest in an adult, 60–70 ml of blood (systolic volume) is ejected into the vascular system with each contraction of the heart, which corresponds to 4–5 liters of cardiac output (the amount of blood ejected by the ventricle in 1 minute). And with heavy physical activity, the minute volume increases to 35 liters and above, while the systolic blood volume can exceed 170 ml, and the systolic blood pressure reaches 200–250 mm Hg. Art.

In addition to blood vessels, there is another type of vessel in the body - lymphatic.

Lymph- a colorless liquid formed from blood plasma by filtering it into the interstitial spaces and from there into the lymphatic system. Lymph contains water, proteins, fats and metabolic products. Thus, the lymphatic system forms an additional drainage system through which tissue fluid flows into the bloodstream. All tissues, with the exception of the superficial layers of the skin, central nervous system and bone tissue, are penetrated by many lymphatic capillaries. These capillaries, unlike blood capillaries, are closed at one end. Lymphatic capillaries are collected into larger lymphatic vessels, which flow into the venous bed in several places. Therefore, the lymphatic system is part of the cardiovascular system.

The most important task of the cardiovascular system is to provide tissues and organs with nutrients and oxygen, as well as remove cell metabolic products (carbon dioxide, urea, creatinine, bilirubin, uric acid, ammonia, etc.). Enrichment with oxygen and removal of carbon dioxide occurs in the capillaries of the pulmonary circulation, and saturation with nutrients in the vessels of the systemic circulation as blood passes through the capillaries of the intestines, liver, adipose tissue and skeletal muscles.

a brief description of

The human circulatory system consists of the heart and blood vessels. Their main function is to ensure blood movement, carried out by working on the principle of a pump. When the ventricles of the heart contract (during their systole), blood is expelled from the left ventricle into the aorta, and from the right into the pulmonary trunk, from which the systemic and pulmonary circulations begin, respectively. The great circle ends with the inferior and superior vena cava, through which venous blood returns to the right atrium. And the small circle contains four pulmonary veins, through which arterial, oxygenated blood flows to the left atrium.

Based on the description, arterial blood flows through the pulmonary veins, which does not correlate with everyday ideas about the human circulatory system (it is believed that venous blood flows through the veins, and arterial blood flows through the arteries).

Having passed through the cavity of the left atrium and ventricle, blood with nutrients and oxygen through the arteries enters the capillaries of the BCC, where oxygen and carbon dioxide are exchanged between it and the cells, nutrients are delivered and metabolic products are removed. The latter, through the bloodstream, reach the excretory organs (kidneys, lungs, gastrointestinal glands, skin) and are excreted from the body.

BKK and MKK are connected to each other in series. The movement of blood in them can be demonstrated using the following diagram: right ventricle → pulmonary trunk → pulmonary vessels → pulmonary veins → left atrium → left ventricle → aorta → systemic vessels → inferior and superior vena cava → right atrium → right ventricle.

Functional classification of vessels

Depending on the function performed and the structural features of the vascular wall, vessels are divided into the following:

1. 1. Shock-absorbing (vessels of the compression chamber) - aorta, pulmonary trunk and large arteries of the elastic type. They smooth out periodic systolic waves of blood flow: they soften the hydrodynamic shock of the blood ejected by the heart during systole, and ensure the movement of blood to the periphery during diastole of the ventricles of the heart.
2. 2. Resistive (vessels of resistance) - small arteries, arterioles, metarterioles. Their walls contain a huge number of smooth muscle cells, thanks to the contraction and relaxation of which they can quickly change the size of their lumen. By providing variable resistance to blood flow, resistive vessels maintain blood pressure (BP), regulate the amount of organ blood flow and hydrostatic pressure in the vessels of the microvasculature (MCR).
3. 3. Exchange - MCR vessels. Through the wall of these vessels, the exchange of organic and inorganic substances, water, and gases occurs between the blood and tissues. Blood flow in the vessels of the MCR is regulated by arterioles, venules and pericytes - smooth muscle cells located outside the precapillaries.
4. 4. Capacitive - veins. These vessels have high distensibility, due to which they can deposit up to 60–75% of the circulating blood volume (CBV), regulating the return of venous blood to the heart. The veins of the liver, skin, lungs and spleen have the greatest depositing properties.
5. 5. Bypass - arteriovenous anastomoses. When they open, arterial blood is discharged along a pressure gradient into the veins, bypassing the MCR vessels. For example, this happens when the skin is cooled, when the blood flow is directed through arteriovenous anastomoses, bypassing the skin capillaries, to reduce heat loss. The skin turns pale.

Pulmonary (lesser) circulation

The ICC serves to saturate the blood with oxygen and remove carbon dioxide from the lungs. After blood enters the pulmonary trunk from the right ventricle, it is sent to the left and right pulmonary arteries. The latter are a continuation of the pulmonary trunk. Each pulmonary artery, after passing through the hilum of the lung, branches into smaller arteries. The latter, in turn, pass into the MCR (arterioles, precapillaries and capillaries). In the MCR, venous blood is converted into arterial blood. The latter enters from the capillaries into venules and veins, which, merging into 4 pulmonary veins (2 from each lung), flow into the left atrium.

Bodily (large) circle of blood circulation

BKK serves to deliver nutrients and oxygen to all organs and tissues and remove carbon dioxide and metabolic products. After blood enters the aorta from the left ventricle, it is directed into the aortic arch. Three branches depart from the latter (brachiocephalic trunk, common carotid and left subclavian arteries), which supply blood to the upper limbs, head and neck.

After this, the aortic arch passes into the descending aorta (thoracic and abdominal). The latter, at the level of the fourth lumbar vertebra, is divided into the common iliac arteries, which supply blood to the lower limbs and pelvic organs. These vessels are divided into external and internal iliac arteries. The external iliac artery passes into the femoral artery, supplying arterial blood to the lower extremities below the inguinal ligament.

All arteries, heading to tissues and organs, in their thickness pass into arterioles and then into capillaries. In the MCR, arterial blood is converted into venous blood. Capillaries become venules and then veins. All veins accompany arteries and are named similarly to arteries, but there are exceptions (portal vein and jugular veins). Approaching the heart, the veins merge into two vessels - the inferior and superior vena cava, which flow into the right atrium.

The extensive network of the human circulatory system consists not only of large veins and arteries, but also of the smallest capillaries, thanks to which all the substances necessary for optimal life are delivered along with blood to each of our cells. It is not surprising that a person’s health largely depends on the state of his cardiovascular system.

Foundation of life

The circulatory system consists of more than just the heart, blood, and blood vessels. This is only one of two complementary systems - cardiovascular and lymphatic. The latter serves to transport lymph, a colorless liquid with many lymphocytes.

The lymphatic system is also extremely important, because human immunity largely depends on it. It is these two systems - cardiovascular and lymphatic - that make up the largest human circulatory system with a total length of more than 100,000 km. This complex mechanism is driven by the heart. This living motor, consisting of muscles, works with amazing performance, pumping more than 9,500 liters of blood per day. In this way, blood is supplied to every cell.

Main functions of the system

The work of the circulatory system begins with the enrichment of blood with oxygen. “Depleted” blood enters the heart through the veins: first into the first chamber of the right atrium, then into the right ventricle of the heart. From there, the more powerful heart muscles push oxygen-deprived blood into the pulmonary trunk, divided into two pulmonary arteries. Next, the blood enters the lungs through numerous pulmonary vessels, where it is enriched with oxygen and returns through the pulmonary vessels to the heart - but this time to the left atrium and ventricle. The left ventricle of the heart is responsible for supplying blood to the entire body, therefore, the muscles of the left ventricle are more developed.

Human blood circulation consists of two circles: small (pulmonary) and large. The small circle is responsible for enriching the blood with oxygen, and the large circle is responsible for transporting blood throughout the body. Despite the fact that two atria and two ventricles contract simultaneously, the thick-walled left ventricle experiences a load six times greater, because it has to circulate blood in a large circle, supplying all limbs with useful substances.

What harms blood vessels?

The scourge of modern man is the deposits of fat on the walls of blood arteries (mainly “bad” cholesterol), as a result of which atherosclerotic changes in blood vessels occur. Fat accumulations form atheromas and cholesterol plaques on the walls of blood vessels, which narrow the patency of blood vessels and impair blood flow. The heart has to work harder, which leads to its premature aging, while little oxygenated blood reaches the tissues. As a result, the body faces the threat of oxygen starvation.

How to keep blood vessels and heart healthy?

The narrowing of the lumen of the arteries leads over time to atherosclerosis, a disease in which the vessels become denser and less elastic. Atherosclerosis can cause even more serious diseases - such as coronary heart disease, hypertension, angina pectoris, myocardial infarction, and so on. These diseases are practically untreatable, so prevention is extremely important for every person.

It is advisable to start improving the circulatory system by changing your lifestyle. This is especially true for overweight people. They should make every effort to normalize it. Moderate and regular physical activity and a proper diet will help you quickly cope with extra pounds, normalize your metabolism and make your circulatory system an effective mechanism for actively supplying the body with all the necessary substances.

As for eating habits, a person striving for healthy heart function should exclude from the diet animal fats, which saturate the body with cholesterol and triglycerides. It is also important to limit the consumption of products such as margarine and palm oil (as a result, most confectionery products). Preference should be given to olive oil and fatty sea fish - products rich in polyunsaturated omega-3 fatty acids.

A healthy circulatory system is a guarantee of your excellent health, vigor and full functioning of all internal organs. Do you want to be healthy? So, take care of the circulatory system!

The distribution of blood throughout the human body is carried out due to the work of the cardiovascular system. Its main organ is the heart. Each blow helps the blood move and nourish all organs and tissues.

System structure

There are different types of blood vessels in the body. Each of them has its own purpose. Thus, the system includes arteries, veins and lymphatic vessels. The first of them are designed to ensure that blood enriched with nutrients flows to tissues and organs. It is saturated with carbon dioxide and various products released during the life of cells, and returns through the veins back to the heart. But before entering this muscular organ, the blood is filtered in the lymphatic vessels.

The total length of the system, consisting of blood and lymphatic vessels, in the adult human body is about 100 thousand km. And the heart is responsible for its normal functioning. It is this that pumps about 9.5 thousand liters of blood every day.

Principle of operation

The circulatory system is designed to provide life support to the entire body. If there are no problems, then it functions as follows. Oxygenated blood exits the left side of the heart through the largest arteries. It spreads throughout the body to all cells through wide vessels and tiny capillaries, which can only be seen under a microscope. It is the blood that enters the tissues and organs.

The place where the arterial and venous systems connect is called the “capillary bed.” The walls of the blood vessels in it are thin, and they themselves are very small. This allows oxygen and various nutrients to be fully released through them. The waste blood enters the veins and returns through them to the right side of the heart. From there it enters the lungs, where it is again enriched with oxygen. Passing through the lymphatic system, the blood is cleansed.

Veins are divided into superficial and deep. The first ones are close to the surface of the skin. They carry blood into the deep veins, which return it to the heart.

Regulation of blood vessels, heart function and general blood flow is carried out by the central nervous system and local chemicals released in the tissues. This helps control the flow of blood through arteries and veins, increasing or decreasing its intensity depending on the processes taking place in the body. For example, it increases with physical activity and decreases with injury.

How does blood flow

The spent “depleted” blood enters the right atrium through the veins, from where it flows into the right ventricle of the heart. With powerful movements, this muscle pushes the incoming fluid into the pulmonary trunk. It is divided into two parts. The blood vessels of the lungs are designed to enrich the blood with oxygen and return it to the left ventricle of the heart. In every person this part of him is more developed. After all, it is the left ventricle that is responsible for how the entire body will be supplied with blood. It is estimated that the load that falls on it is 6 times greater than that to which the right ventricle is exposed.

The circulatory system includes two circles: small and large. The first of them is designed to saturate the blood with oxygen, and the second is to transport it throughout the orgasm, delivering it to every cell.

Requirements for the circulatory system

In order for the human body to function normally, a number of conditions must be met. First of all, attention is paid to the condition of the heart muscle. After all, it is the pump that drives the necessary biological fluid through the arteries. If the functioning of the heart and blood vessels is impaired, the muscle is weakened, this can cause peripheral edema.

It is important that the difference between low and high pressure areas be maintained. This is necessary for normal blood flow. For example, in the area of ​​the heart the pressure is lower than at the level of the capillary bed. This allows you to comply with the laws of physics. Blood moves from an area of ​​higher pressure to an area where it is lower. If a number of diseases arise due to which the established balance is disturbed, then this is fraught with stagnation in the veins and swelling.

The release of blood from the lower extremities is carried out thanks to the so-called muscular-venous pumps. This is the name of the calf muscles. With each step, they contract and push blood against the natural force of gravity towards the right atrium. If this functioning is disrupted, for example, as a result of injury and temporary immobilization of the legs, then edema occurs due to a decrease in venous return.

Another important link responsible for ensuring that human blood vessels function normally are the venous valves. They are designed to support fluid flowing through them until it enters the right atrium. If this mechanism is disrupted, perhaps as a result of injury or due to wear and tear of the valves, abnormal blood collection will occur. As a result, this leads to an increase in pressure in the veins and squeezing out the liquid part of the blood into the surrounding tissues. A striking example of a violation of this function is varicose veins in the legs.

Classification of vessels

To understand how the circulatory system works, you need to understand how each of its components functions. Thus, the pulmonary and vena cava, pulmonary trunk and aorta are the main routes for the movement of the necessary biological fluid. And everyone else is able to regulate the intensity of blood inflow and outflow to tissues due to the ability to change their lumen.

All vessels in the body are divided into arteries, arterioles, capillaries, venules, and veins. They all form a closed connecting system and serve a single purpose. Moreover, each blood vessel has its own purpose.

Arteries

The areas through which blood moves are divided depending on the direction in which it moves in them. So, all arteries are designed to transport blood from the heart throughout the body. They come in elastic, muscle and muscle-elastic types.

The first type includes those vessels that are directly connected to the heart and emerge from its ventricles. These are the pulmonary trunk, pulmonary and carotid arteries, and aorta.

All of these vessels of the circulatory system consist of elastic fibers that stretch. This happens with every heartbeat. As soon as the contraction of the ventricle has passed, the walls return to their original form. Due to this, normal pressure is maintained for a period until the heart fills with blood again.

Blood enters all tissues of the body through arteries that arise from the aorta and pulmonary trunk. At the same time, different organs need different amounts of blood. This means that the arteries must be able to narrow or expand their lumen so that fluid passes through them only in the required doses. This is achieved due to the fact that smooth muscle cells work in them. Such human blood vessels are called distributive. Their lumen is regulated by the sympathetic nervous system. Muscular arteries include the cerebral artery, radial, brachial, popliteal, vertebral and others.

Other types of blood vessels are also distinguished. These include muscular-elastic or mixed arteries. They can contract very well, but are also highly elastic. This type includes the subclavian, femoral, iliac, mesenteric arteries, and celiac trunk. They contain both elastic fibers and muscle cells.

Arterioles and capillaries

As blood moves along the arteries, their lumen decreases and the walls become thinner. Gradually they turn into the smallest capillaries. The area where the arteries end is called arterioles. Their walls consist of three layers, but they are poorly defined.

The thinnest vessels are capillaries. Together they represent the longest part of the entire circulatory system. They are the ones that connect the venous and arterial beds.

A true capillary is a blood vessel that is formed as a result of the branching of arterioles. They can form loops, networks that are located in the skin or synovial bursae, or vascular glomeruli located in the kidneys. The size of their lumen, the speed of blood flow in them and the shape of the networks formed depend on the tissues and organs in which they are located. For example, the thinnest vessels are located in skeletal muscles, lungs and nerve sheaths - their thickness does not exceed 6 microns. They form only flat networks. In mucous membranes and skin they can reach 11 microns. In them, the vessels form a three-dimensional network. The widest capillaries are located in the hematopoietic organs and endocrine glands. Their diameter reaches 30 microns.

The density of their placement is also uneven. The highest concentration of capillaries is observed in the myocardium and brain; for every 1 mm 3 there are up to 3,000 of them. At the same time, in skeletal muscle there are only up to 1,000 of them, and in bone tissue even less. It is also important to know that in an active state, under normal conditions, blood does not circulate through all capillaries. About 50% of them are in an inactive state, their lumen is compressed to a minimum, only plasma passes through them.

Venules and veins

Capillaries, into which blood flows from arterioles, unite and form larger vessels. They are called postcapillary venules. The diameter of each such vessel does not exceed 30 microns. At the transition points, folds are formed that perform the same functions as valves in the veins. Blood elements and plasma can pass through their walls. Postcapillary venules unite and flow into collecting venules. Their thickness is up to 50 microns. Smooth muscle cells begin to appear in their walls, but often they do not even surround the lumen of the vessel, but their outer membrane is already clearly defined. The collecting venules become muscular. The diameter of the latter often reaches 100 microns. They already have up to 2 layers of muscle cells.

The circulatory system is designed in such a way that the number of vessels draining blood is usually twice as large as the number of those through which it enters the capillary bed. In this case, the liquid is distributed like this. The arteries contain up to 15% of the total amount of blood in the body, the capillaries contain up to 12%, and the venous system contains 70-80%.

By the way, fluid can flow from arterioles to venules without entering the capillary bed through special anastomoses, the walls of which include muscle cells. They are found in almost all organs and are designed to allow blood to be discharged into the venous bed. With their help, pressure is controlled, the transition of tissue fluid and blood flow through the organ are regulated.

Veins are formed after the fusion of venules. Their structure directly depends on location and diameter. The number of muscle cells is influenced by their location and the factors under which fluid moves into them. Veins are divided into muscular and fibrous. The latter include the vessels of the retina, spleen, bones, placenta, soft and hard membranes of the brain. The blood circulating in the upper part of the body moves mainly under the force of gravity, as well as under the influence of the suction action during inhalation of the chest cavity.

The veins of the lower extremities are different. Each blood vessel in the legs must withstand the pressure created by the column of fluid. And if the deep veins are able to maintain their structure due to the pressure of the surrounding muscles, then the superficial ones have a more difficult time. They have a well-developed muscle layer, and their walls are much thicker.

Another characteristic feature of veins is the presence of valves that prevent the reverse flow of blood under the influence of gravity. True, they are not in those vessels that are located in the head, brain, neck and internal organs. They are also absent in the hollow and small veins.

The functions of blood vessels vary depending on their purpose. So, veins, for example, serve not only to move fluid to the heart area. They are also designed to reserve it in separate areas. Veins are used when the body works hard and needs to increase the volume of circulating blood.

Structure of arterial walls

Each blood vessel consists of several layers. Their thickness and density depend solely on what type of veins or arteries they belong to. This also affects their composition.

For example, elastic arteries contain a large number of fibers that provide stretching and elasticity of the walls. The inner lining of each such blood vessel, which is called the intima, makes up about 20% of the total thickness. It is lined with endothelium, and underneath there is loose connective tissue, intercellular substance, macrophages, and muscle cells. The outer layer of the intima is limited by an internal elastic membrane.

The middle layer of such arteries consists of elastic membranes; with age they thicken and their number increases. Between them are smooth muscle cells that produce intercellular substance, collagen, and elastin.

The outer shell of the elastic arteries is formed by fibrous and loose connective tissue; elastic and collagen fibers are located longitudinally in it. It also contains small vessels and nerve trunks. They are responsible for feeding the outer and middle shells. It is the outer part that protects the arteries from ruptures and overextensions.

The structure of the blood vessels, which are called muscle arteries, is not much different. They also consist of three layers. The inner shell is lined with endothelium, it contains an internal membrane and loose connective tissue. In small arteries this layer is poorly developed. Connective tissue contains elastic and collagen fibers, they are located longitudinally in it.

The middle layer is formed by smooth muscle cells. They are responsible for contracting the entire vessel and pushing blood into the capillaries. Smooth muscle cells connect with the intercellular substance and elastic fibers. The layer is surrounded by a kind of elastic membrane. The fibers located in the muscle layer are connected to the outer and inner membranes of the layer. They seem to form an elastic frame that prevents the artery from sticking together. And muscle cells are responsible for regulating the thickness of the lumen of the vessel.

The outer layer consists of loose connective tissue, which contains collagen and elastic fibers; they are located obliquely and longitudinally in it. It also contains nerves, lymphatic and blood vessels.

The structure of mixed type blood vessels is an intermediate link between muscular and elastic arteries.

Arterioles also consist of three layers. But they are expressed rather weakly. The inner shell is the endothelium, a layer of connective tissue and elastic membrane. The middle layer consists of 1 or 2 layers of muscle cells that are arranged in a spiral.

Vein structure

In order for the heart and blood vessels called arteries to function, it is necessary that the blood can flow back up, bypassing the force of gravity. Venules and veins, which have a special structure, are intended for these purposes. These vessels consist of three layers, just like arteries, although they are much thinner.

The inner lining of the veins contains endothelium, it also has a poorly developed elastic membrane and connective tissue. The middle layer is muscular, it is poorly developed, and there are practically no elastic fibers in it. By the way, it is precisely because of this that the cut vein always collapses. The outer shell is the thickest. It consists of connective tissue and contains a large number of collagen cells. It also contains smooth muscle cells in some veins. They help push blood towards the heart and prevent it from flowing back. The outer layer also contains lymphatic capillaries.